Scientists continue to strive to find ways to reduce the cell size of liquid crystal displays for a variety of developing applications. This becomes important when considering the small cell size dictated by monochromatic head-mounted or optical projection displays required for a UXGA form factor. These require provision of 1600.times.1280 pixels with a cell size of 18.times.18 .mu.m.sup.2. A comparable colored display requires a cell size of 6.times.18 .mu.m.sup.2 for each primary color. Display cell structures that are based on the transmissive liquid crystal technologies are well developed and their continued use allow the scientists to take advantage of their simple and low cost optics. In these prior art display techniques, as the display cell size is made to shrink to less than 20 microns the LCD electrode width becomes comparable in size to the LCD layer thickness. This layer thickness provides the gap between the LCD control electrodes. In this situation, the ability to regulate the light transmitting properties of each pixel becomes less and less controllable. This is due to the 3D-effect inherent in liquid crystals. The 3D-effect causes irregular switching control when the LCD ratio of electrode width to gap thickness becomes small. Typical displays employ a safe LCD ratio of greater than five to one to effectively eliminate the 3D-effect.
FIG. 1 shows a pixel within a typically assembled liquid crystal display 100. It shows a lower substrate 130 upon which the semiconductor circuit 132 and the lower electrode 135 is formed. The upper substrate 110 is separated from the lower substrate 130 by a liquid crystal layer 120. The liquid crystal layer 120 is covered with the upper electrode 115. The liquid crystal ON/OFF state control voltage is connected across the lower 135 and upper 115 electrodes. When the control voltage is present the liquid crystal is in one light transmitting state, and when the control voltage is not present the liquid crystal is in the opposite light transmitting state.
In prior art liquid crystals the thickness `y` 140 of the liquid crystal layer 120 is generally greater than 6-8 microns. The width `x` 150 of the horizontal lower layer electrode 135 is usually greater than 40 microns. This results in an LCD pixel aspect ratio of at least 5:1. In order to achieve a smaller size pixel using this technique, the width `x` 150 of the horizontal lower layer electrode 135 must be reduced. This leads to the 3D effects problem in controlling the ON/OFF state of the liquid crystal.
Typically a PDLC material requires a cell gap thickness of 6 to 8 microns so that light passing through the material is sufficiently scattered in the OFF state. Prior art techniques use horizontal electrodes. In this case, as the width of the electrodes is reduced the ratio of the electrode width `x` 150, to cell gap thickness `y` 140, becomes perilously small. For example, a 6 micron width electrode with a 6 micron cell gap seriously suffers from the aforementioned 3D electrical field effects. In this situation, the cell does not completely switch from the `OFF` to the `ON` state. Also, near the edges of the cell insufficient field strength exists to adequately align the PDLC material to efficiently pass the light. Additionally, light passing through the cell at angles off the display normal are scattered and lost due to the insufficiency of the field strength.
The present invention overcomes this problem by using vertical electrodes. Back-end-of-the-line vertical cells are built on top of the x, y lines. Each vertical cell is filled with PDLC (polymer dispersed liquid crystal). By forming and aligning the electrodes in a vertical manner the 3D-effect problem is solved in a way that allows small pixel sizes to be used. This is achieved without loss of pixel contrast while still maintaining relatively complete switching from dark to bright states. Another problem overcome by the present invention is sustaining the efficiency of light transmission through each pixel. This is accomplished by maintaining an aperture ratio of greater than 20 percent even when the pixel edges are in the order of 6 microns transmitting the light.
As cell size is reduced by the use of vertical electrodes another problem becomes apparent. This is the problem that the percentage of clear pixel area or aperture ratio of the prior art designs becomes unacceptably low. This is due to the light obscuring properties of the opaque areas within each pixel taken up by the row and column lines, the thin-film-transistor and the storage capacitor.
A display should provide an aperture ratio of greater than 30%. In liquid crystal displays having relatively large pixels, the storage capacitor obscures about 10 to 20% of the region through which light is transmitted in the transmissive display. As the pixel cell size is reduced the percentage of area obscured by the storage capacitor becomes intolerable. It causes a severe reduction in the amount of available light transmitted through the pixel.
None of the known existing transmissive liquid crystal cells on poly-Si or c-Si can be scaled to such small cell sizes while maintaining a greater than 30 percent aperture ratio. Two alternatives have been suggested. One method is to use reflective cells. This approach requires more expensive optical components. The other approach calls for self-luminous cells such as the LED, electroluminescence, or organic-LED cells. All self-luminous cells face the differential aging problem that is a material-related unknown at the present time.
One aspect of the present invention also solves this problem by using a vertical trench capacitor which is hidden under the row and column lines. In this way, the opaque area of each pixel is limited to the areas taken up by a single transistor and the row and column x,y lines.